This specification describes technologies relating to controlling optical amplifiers. In one implementation, an optical amplifier is provided. The optical amplifier includes a light amplifying medium for receiving an input optical signal and outputting an output amplified signal; a first measuring block for measuring a change in power of the input signal; a pump laser for supplying pump light to the light amplifying medium; and an electronic control for controlling the power of the pump light in response to the measured change in power of input signal to provide an output amplified signal having a substantially constant power for one or more changes in the power of the input signal.